Ultrastructural investigations of presumed photoreceptive organs in two Saccocirrus species (polychaeta,saccocirridae)

In addition to the pigmented ocelli, four different types of photoreceptor-like organs without shading pigment have been found in Saccocirrus papillocercus and S. krusadensis. The sensory cells of these presumed ocelli are either ciliary or rhabdomeric with ciliary rudiments. With the exception of the multicellular type-2 ocelli they are bicellular consisting of a sensory cell and a supportive cell. In each ocellus the supportive cell forms a thin cup-shaped envelope around the sensory elements. In the type-2 ocellus, 7 supportive cells form an ovoid cavity leaving openings through which dendritic processes of an equal number of sensory cells enter the cavity. The pigmented ocelli possess an ocellar cavity communicating with the exterior through a pore in the eyecup, ciliary rudiments in both sensory and supportive cell, and additional non-photoreceptive sensory cells in the opening of the eyecup. The sensory organs show characteristic differences between the two species, such as presence or absence of a particular type of ocellus (type 2 is absent in S. krusadensis, type 3 in S. papillocercus), number of cilia in type-4 ocelli, density of microvilli, number of non-photoreceptive sensory cells in the pore of the pigmented ocellus, etc. These differences provide important characters which can be used for discrimination either of species or of subgeneric taxa in Saccocirrus. The phylogenetic significance of the different photoreceptive organs is discussed.     

Ultrastructural investigations of presumed photoreceptors as a means of discrimination and identification of closely related species of the genus Microphthalmus (Polychaeta,Hesionidae)

Summary Differences in the ultrastructure of presumed photoreceptors of three morphologically similar Microphthalmus populations on the opposite sides of the Atlantic (German North Sea coast and coasts of North Carolina and Massachusetts) suggest the existence of three different species. Only the European M. listensis possesses three pairs of prostomial eyes, of which one pair has rhabdomeric receptors and pigment cells. The two other pairs are unpigmented and can be found in all three species. The frontal one has ciliary receptors, the posterior one rhabdomeric sensory cells. An additional unpaired potential photoreceptor organ in the segment with the first pair of tentacular cirri is present in all individuals of this species complex. It has a relatively high number of cilia with numerous microvillar projections. — For each type of ocellus there are slight but distinct and constant differences among the species such as relative position of sensory cells, presence of dilations of the ciliary shafts, number of cilia, and shape of the sensory cells. Presence of both ciliary and rhabdomeric light-sensitive cells is discussed with reference to various theories of the evolution of photoreceptors.Abbreviations ax axonema - bb basal body - cc cup cell - ci cilium - cu cuticle - epc epidermal cell - g Golgi apparatus - gp glycogen particles - mi mitochondrion - mv microvilli - mvb multivesicular body - nu nucleus - pc pigment cell - pg pigment granule - rer rough ER - smc submicrovillar cysternae - sr striated rootlet     

Evolution of cerebral vesicles and their sensory organs in an ascidian larva

Sorrentino M., Manni L., Lane N. J. and Burighel P. 2000. Evolution of cerebral vesicles and their sensory organs in an ascidian larva. —Acta Zoologica (Stockholm) 81 : 243–258 The ascidian larval nervous system consists of the brain (comprising the visceral ganglion and the sensory vesicle), and, continuous with it, a caudal nerve cord. In most species two organs, a statocyst and an ocellus with ciliary photoreceptors, are contained in the sensory vesicle. A third presumptive sensory organ was sometimes found in an ‘auxiliary’ ganglionic vesicle. The development and morphology of the sensory and auxiliary ganglionic vesicles in Botryllus schlosseri and their associated organs was studied. The sensory vesicle contains a unique organ, the photolith, responding to both gravity and light. It consists of a unicellular statocyst, in the form of an expanded pigment cup receiving six photoreceptor cell extensions. Presumptive mechano‐receptor cells (S1 cells), send ciliary and microvillar protrusions to contact the pigment cup. A second group of distinctive cells (S2), slightly dorsal to the S1 cells, have characteristic microvillar extensions, resembling photoreceptor. We concur with the idea that the photolith is new and derived from a primitive statocyst and the S2 cells are the remnant of a primitive ocellus. In the ganglionic vesicle some cells contain modified cilia and microvillar extensions, which resemble the photoreceptor endings of the photolith. Our results are discussed in the light of two possible scenarios regarding the evolution of the nervous system of protochordates.     

The larval ocelli of Golfingia misakiana (Sipuncula, Golfingiidae) and of a pelagosphera of another unidentified species

 The inverse cerebral ocelli of the pelagosphera larva of Golfingia misakiana and of another unidentified larva are composed of two or three sensory cells and one supportive pigmented cell. The sensory cells bear an array of microvilli as well as a single cilium with poor undulation of its membrane; the photoreceptive organelles are regarded as the rhabdomeric type. A striking feature of these cells is the cores, which extend within the microvilli from the tip into the midregion of the cell. It is suggested that these structures are identical with the submicrovillar cisternae found in the cerebral inverse eyes of larvae of Polychaeta. The findings allow the conclusion that in the pelagosphera of the Sipuncula, contrary to the teleplanic veliger larvae of Gastropoda, a lengthy pelagic cycle is not correlated with the development of a ciliary photoreceptor. Additionally, it is assumed that the pigment cup ocelli in larvae of Sipuncula are homologous with the cerebral inverted pigment cup ocelli of larvae of Polychaeta. Accepted: 19 March 1997     

The ultrastructure of the eyes in the veliger-larvae of Aporrhais sp. and Bittium reticulatum (Mollusca,Caenogastropoda)

Summary The cerebrally innervated larval eyes of Aporrhais sp. and Bittium reticulatum are investigated by means of transmission electron microscopy. Each organ consists of a pigmented cup containing an acellular lens. The cornea overlaps the anterior portion of the eye. The retina is composed of sensory cells and supportive cells. The sensory cells of Aporrhais sp. bear one cilium and in Bittium reticulatum two cilia, the ciliary membrane being folded into numerous finger-shaped evaginations. The supportive cells contain the pigment granules and most of them bear one or two cilia, the plasmalemma of which is likewise folded. It is supposed that: (a) these cilia have a transportive function for lens material and (b) that the ciliary photoreceptor of Aporrhais sp. and Bittium reticulatum is a functional adaptation to a relatively long larval period.Abbreviations bb basal body - bp basal plate - c cilium - cc corneal cell - cm ciliary membranes - cw ciliary whorl - gd Golgi dictyosomes - gm granular material - l lens - m mitochondrion - mt microtubules - mv microvilli - mvb multivesicular body - n nucleus - pb pigment border - pg pigment granule - rer rough endoplasmic reticulum - sc sensory cell - sj septate junctions - spc supportive cell     

Fine structure of the epidermis in Gnathostomulida

Summary The fine structure of the epidermis in Haplognathia simplex, Haplognathia rosea, Pterognathia meixneri (Filospermoidea) and Gnathostomula paradoxa (Bursovaginoidea) has been investigated. The epidermis in the filospermoidean species is uniform, consisting of epidermal cells with a single locomotory cilium. The structure and development, including ciliogenesis, of these epidermal cells are described. In G. paradoxa additional epidermal elements have been found: mucous cells with a presumably apocrine secretion modus are scattered in a strip-like arrangement within the epidermis. Their deverlopment is separate from epidermal cells with locomotory function. Two further types of glandular cells with either a single cilium or a diplosome are located ventrally. It is assumed that they represent an adhesive system.Abbreviations (used in figures) ac accessory centriole - ap appendix of accessory centriole - ax axoneme - bb basal body - bf basal foot - bl basal lamina - c cilium - cA ciliary adhesive cell - ce centriole - cp ciliary pit - d diplosome - dy dictyosome - dA diplosomal adhesive cell - E epidermal cell with locomotory cilium, epidermis - ev epidermal vesicle (epitheliosome) - gv gland vesicle - m mitochondrion - ma microvillus of apical cell membrane - mp microvillus of ciliary pit - mv microvillus - n nucleus - ps prosecretory-vesicle - R receptor - r ciliary rootlet - rc caudal ciliary rootlet - rr rostral ciliary rootlert - sv secretory vesicle - v vesicle - v _i central vesicle of multivesicular body - v _o surrounding vesicle of multivesicular body - z cisternae     

Sensory endings of the ascidian static organ (Chordata,Ascidiacea)

Summary The ultrastructure of the static organ is examined in larvae of Diplosoma macdonaldi, a colonial ascidian, and Styela plicata, a solitary ascidian; the results are similar. As previous workers found, the cell body of a unicellular statocyte lies in the lumen of the sensory vesicle and contains the statolith. A narrow neck connects the cell body to an anchoring foot in the floor of the sensory vesicle. Two previously undescribed sensory endings project into the lumen just to the left of the statocyte, one anterior and one posterior to the neck. A network of fine processes from each ending contacts the statocyte body. It is proposed that movements of the statocyte cell body are detected by these endings. They arise from neurons in the ventral wall of the sensory vesicle that project axons to the visceral ganglion. The placement of the sensory endings may allow discrimination of the directon of statocyte deflection.Abbreviations ax axons - bb ciliary basal body - bl basal lamina - c cilium - cr striated ciliary rootlet - ec ependymal cells - en endoderm - h hemocoel - ly lysosome - mv microvilli - n neuron - nf neurofilaments - ns neck of the statocyte - sb statocyte cell body - sd sensory dendrite - sn sensory neuron - sp sensory processes - stf statocyte foot - svl sensory vesicle lumen - zo zonula occludens     

Comments on the eyes of tardigrades

A survey is given on the scarce information on the visual organs (eyes or ocelli) of Tardigrada. Many Eutardigrada and some Arthrotardigrada, namely the Echiniscidae, possess inverse pigment-cup ocelli, which are located in the outer lobe of the brain, and probably are of cerebral origin. Occurrence of such organs in tardigrades, suggested as being eyeless, has never been checked. Depending on the species, response to light (photokinesis) is negative, positive or indifferent, and may change during the ontogeny. The tardigrade eyes of the two eutardigrades examined up to now comprise a single pigment cup cell, one or two microvillous (rhabdomeric) sensory cells and ciliary sensory cell(s). In the eyes of the eutardigrade Milnesium tardigradum the cilia are differentiated in an outer branching segment and an inner (dendritic) segment. Because of the scarcity of information on the tardigrade eyes, their homology with the visual organs of other bilaterians is currently difficult to establish and further comparative studies are needed. Thus, the significance of these eyes for the evolution of arthropod visual systems is unclear yet.     

First evidence of mitochondrial lensing in two species of the “Typhloplanoida” (Plathelminthes,Rhabdocoela): phylogenetic implications

 Based on electron-microscopical observations the light-sensing organs of Proxenetes deltoides and Ptychopera westbladi, representatives of the ”Typhloplanoida” Trigonostominae, are described. The photoreceptors in both species belong to the type of rhabdomeric pigment cup ocelli. P. deltoides has a single pigment cell and three sensory cells. P. westbladi possesses eyes made up of a single pigmented cup cell and a single sensory cell. The dioptric apparatus in the eyes of P. deltoides is formed by three proliferations of the cup cell containing giant mitochondria. In P. westbladi, the elements focalizing incoming light also consist of modified mitochondria which are arranged in the section of the cup cell covering the eye cavity. With regard to the new findings, mitochondrial lensing is hypothesized as an autapomorphy of a monophylum encompassing distinct taxa or all members of the free-living Rhabdocoela; the Neodermata also belong to this monophylum. Accepted: 21 March 1996     

Ultrastructure of the cephalic sensory organs of adult Pycnophyes dentatus and of the first juvenile stage of P. kielensis (Kinorhyncha, Homalorhagida)

 The ultrastructure of the paired cephalic sensory organs of adult Pycnophyes dentatus and of the first juvenile stage of P. kielensis (Kinorhyncha, Homalorhagida) was investigated by TEM. In both species, each sensory organ is composed of one receptor cell and one enveloping cell which border a common intercellular lumen. A single receptor cilium extends from the receptor cell into this lumen. The cilium expands behind the basal body and branches into numerous processes. A pair of cephalic sensory organs with these characteristics belongs to the ground pattern of, at least, the Pycnophyidae. The sensory organs of these Kinorhyncha correspond closely with the anterior cephalic organs of the Gastrotricha, but differ from the known cephalic receptors of other Nemathelminthes. Currently, it cannot be evaluated conclusively whether the last common ancestor of the Nemathelminthes possessed cephalic sensory organs and, if it did, what these organs looked like. Accepted: 3 December 1996     

Ultrastructure of prostomial photoreceptors in four marine polychaete species (annelida)

The photoreceptors of four polychaete species were investigated by transmission electron microscopy: Eteone longa and Anaitides mucosa (Phyllodocidae), Scolelepis squamata (Spionidae), and Heteromastus filiformis (Capitellidae). Four different types of light-sensitive organs could be distinguished: 1) a simple, unpigmented rhabdomeric type; 2) a simple ocellus composed of a sensory and a pigmented cell; 3) complex eyes with a lens consisting of secretory granules; 4) a simple, unpigmented type with modified cilia. In spite of its simpler organization the fourth type is listed last, because its function as a photoreceptor seems dubious. The first type (unpigmented rhabdomeric receptor) occurs in all four species investigated. It is the only type of photoreceptor in Heteromastus. Additionally, the two phyllodocids Eteone and Anaitides possess another kind of receptor (type 4) in close proximity to the type 1 receptor. Simple ocelli (type 2) are found in Scolelepis. A pair of complex eyes (type 3) is present in both Eteone and Anaitides, but they show important differences in the two species. First, the eyes in Eteone exhibit ciliary rudiments within the sensory processes, but such rudiments are absent in the eyes of Anaitides. Secondly, the sensory cells in Anaitides possess pigment granules, whereas in Eteone they do not. Thirdly, the lens in Eteone is composed of secretion granules of equal electron density, whereas in Anaitides the lens granules show increased electron density centrally. Lens material appears to be secreted from a single corneal cell in Eteone, and from several corneal cells in Anaitides. In both species these corneal cells are located distally outside the lens.     

Ultrastructure of copulatory organs in Turbellaria

Summary The copulatory organs in Macrostomum sp. and Microstomum sp. contain simple tubular stylets which are intracellular specializations. The stylet in Macrostomum sp. is produced in a syncytium covering part of the prostatic vesicle. The proximal region of the stylet surrounds the vesicle which contains six prostatic gland ducts and six accessory (sensory) cells containing ciliary rootlets. The stylet in Microstomum sp. is produced in an extension of a syncytium which lines the combined seminal-prostatic vesicle. The stylet is connected to the combined vesicle by a narrow bridge of matrix syncytium through which sperm, prostatic gland products and sensory cilia pass from the vesicle to the stylet lumen. In both species the matrix syncytium can be interpreted as a specialized terminal end of the male canal epithelium. Stylets of Turbellaria and other lower Metazoa are discussed in regards to structure (one or several pieces) and location (in separate cells, in a syncytium, or extracellular).Abbreviations used in figures ac accessory cell - b basal body - c cilium - cv combined vesicle - d prostatic gland duct - dc degenerative cell - di dictyosome - e epidermis - ed ejaculatory duct - g prostatic gland cell - h hemidesmosome - i intercellular matrix - im internal muscle - in intestine; - l lumen of male canal - lm longitudinal muscle - m matrix syncytium - mc male canal epithelial cell - mi microfilaments - mt microtubules - mu muscle cell - mv microvilli - n nucleus - np nerve process - ns neurosecretory (?) granule - p prostatic vesicle - pv prostatic part of combined vesicle - r rootlet - s stylet - sm stylet material - sp sperm - sv seminal part of combined vesicle     

The Nymphal Eyes of Parabuthus transvaalicus Purcell, 1899 (Buthidae): An Accessory Lateral Eye in a Scorpion

In P. transvaalicus nymphs, 5 pairs of lateral ocelli each composed of a corneal lens, R-cell units forming a latticed rhabdom, arhabdomeric cells and pigment cells are present. In addition, we found a pair of unpigmented accessory sense organs situated ventroposteriorly to the lateral ocelli in prenymphs as well as in first nymphs. They are composed of primary, rhabdomeric sensory cells, and we infer that they represent a second type of lateral eye. They also comprise sensory units, but lenses and screening pigment are lacking. Their position and cellular architecture corresponds well with that of the “rudimentary” lateral eye of the xiphosuran, Limulus. The occurrence of a bipartite lateral visual system in Chelicerata and Arthropoda is discussed.     

Ultrastructural differences as a taxonomic marker: the segmental ocelli of Polyophthalmus pictus and Polyophthalmus qingdaoensis sp.n. (Polychaeta,Opheliidae)

The segmental ocelli (eyes) in specimens of a European and a Chinese Polyophthalmus pictus population have been investigated by transmission electron microscopy. The ocelli are situated in corresponding positions in the same segments and reveal similarities in their general structure. They consist of one photoreceptor cell with microvilli-bearing processes and a pigment cup, the receptor processes project into an extracellular cavity formed by the sensory cell and a few supporting cells, the pigment cup is formed by mesodermal cells, and basiepidermal glial cells and gland cells lie above the sensory cell. However, the ocelli differ in size and number of cells, number and dimensions of cellular elements as well as presence or absence of certain cell types associated with the ocelli. There is only little variation in these characters and there is no overlap, so that they distinctly separate the specimens of the two populations. These differences are in the same range, or even larger, as those observed between the ocelli of other closely related polychaete species. Therefore, the specimens from Qingdao, China, are described as a new species of the Opheliidae, Polyophthalmus qingdaoensis sp.n., although specimens from Qingdao, China, and the Island of Giglio, Italy, are almost inseparable by light microscopy except for a few subtle differences.     

Ultrastructure of presumptive light sensitive ciliary organs in larvae of Poecilochaetidae,Trochochaetidae, Spionidae,Magelonidae (Annelida) and its phylogenetic significance

Larvae of Poecilochaetus serpens, Trochochaeta multisetosum and Polydora ciliata possess almost identical unpigmented, ciliary, presumptive light sensitive organs within the prostomium. The data corroborate hypotheses on the close relationship of Poecilochaetidae, Trochochaetidae and Spionidae and are even congruent with inclusion of Poecilochaetidae and Trochochaetidae within Spionidae. The organs in P. serpens, T. multisetosum and P. ciliata are composed of one monociliary receptor cell, one supportive cell and several associated flask shaped bipolar sensory cells. The receptor cell cilium enters the supportive cell cavity through a thin pore, dilates and then branches into a high number of disordered projections. The associated sensory cells bear one or occasionally two cilia, which run horizontally beneath or within the cuticle. The supportive cell cavity is not sealed by any cell contact from the subcuticular extracellular space. The organs in Magelona mirabilis are composed of a single supportive cell, but several receptor cells. No further sensory cells are associated. Each receptor cell sends one cilium into an own invagination of the supportive cell, and the ciliary branches are highly ordered. The examined organs in P. serpens, T. multisetosum and P. ciliata exhibit a unique organization amongst polychaetes. The organs of M. mirabilis are most probably homologous. A homology to ciliary organs of Protodrilida is conceivable. In the lineage leading to Protodrilida, primary larval organs may have been integrated into the adult body organization by heterochrony.     

Comparative morphology of photoreceptors in free-living plathelminths — a survey

In free-living Plathelminthes, the best-known photoreceptors are pigment-cup ocelli, eyes formed of one or several pigmented supportive cells into whose cup-shaped cavity project the light-sensitive elements of one or several sensory cells. Besides these, so-called Sehkolben, photoreceptors lacking pigment granules, are found in some species. Sensory cells in plathelminth photoreceptors most commonly use microvilli as the light-sensitive organelles, but some use cilia and combinations of microvilli and cilia. Lamellate ciliary bodies with cilia whose membranes are strongly flattened and rolled and pericerebral ciliary aggregations with interwoven cilia protruding into an intracellular cavity are likely photoreceptors in that they show amplification of membrane likely to bear photoreceptive pigments. Cells with ballooned cilia and tubular vacuoles are other differentiations to which light-sensitivity has been attributed. A variety of structures serve as lenses, all usually formed from parts of the pigment cell.     

Nervous system of ascidian larvae: Caudal primary sensory neurons

Summary In larvae of Diplosoma macdonaldi one sensory nerve extends along the dorsal midline of the tail and another extends along the ventral midline. Each nerve is composed of 50–70 naked axons lying in a groove in the base of the epidermis, and each projects to the visceral ganglion. The cell bodies of the caudal sensory neurons occur in pairs within the epidermis, and are situated along the courses of the nerves. A single cilium arises from an invagination in the soma of each neuron, passes through the inner cuticular layer of the tunic and enters a tail fin formed by the outer cuticular layer. We propose that these cells are mechanoreceptors. The caudal sensory system is similar in representative species of ten families of ascidians.Abbreviations a axial complex of the tail - ac accessory centriole - ax axon - bb basal body - bl basal lamina - c cilium - cep common epidermal cells - cs ciliary sheath - dcv dense-cored vesicles - dsn dorsal sensory nerve - ec ependymal cells - ep epidermis - gj gap junction - h hemocoel - hc hemocoelic chamber - icl inner cuticular layer of the tunic - m caudal muscle - nc dorsal nerve cord - ncl neurocoel - no notochord - ocl outer cuticular layer of the tunic - sc sensory cell - sn sensory nerve - sv sensory vesicle - vg visceral ganglion - vsn ventral sensory nerve     

A new glandular sensory organ inCatanema sp. (Nematoda,Stilbonematinae)

Summary A new multicellular glandular sensory organ is described forCatanema sp. (Nematoda, Stilbonematinae). The organs terminate in setae and are distributed in six longitudinal rows along the body. Two types of glandular cells (type A and type B), one monociliary sensory cell and one undifferentiated epidermal cell are combined in the basiepidermal organ. A comparison of epidermal glands as well as sensory organs in Nematoda is made. A causal relationship between the development of such complex, large and numerous glandular sensory organs and the occurrence of species-specific, symbiotic epibacteria inCatanema sp. seems probable, although there is no simple correlation between the distribution of these organs and epibacteria. A mucous cover over the bacterial layer, released by the glandular sensory organs, may create a microenvironment for the interaction between epibionts and host.Abbreviations (used in figures) a amphid - A1–A4 type 1–4 granules of type A gland cell - an annuli - b bacteria - B1–B3 type 1–3 granules of type B gland cell - bl basal lamina - bp basal part of seta - bz basal zone of cuticle - c cuticle - ca canal - cg caudal gland - ci cilium - cz cortical zone of cuticle - d dictyosomes - e epidermis - e co extracellular coat - em extracellular matrix - ep epicuticle - f filaments - gcA type 1 gland cell - gcB type 2 gland cell - i lp inner labial papillae - m mitochondrion - me membranes of type 2 gland cell - mo mouth opening - mz median zone of cuticle - n nucleus - nu nucleolus - p process - pv primary vesicle of type A gland cell - r ribosomes - s seta - sc sensory cell - sp secretory product - tj tight junction - tp terminal part of seta - uc undifferentiated epidermal cell - va vacuoles or vesicles of epidermal cells - ve vesicles of sensory cell     

The ciliary photoreceptor in the teleplanic veliger larvae of Smaragdia sp. and Strombus sp. (Mollusca,Gastropoda)

The cerebrally innervated eyes of the veliger larvae of Smaragdia sp. and Strombus sp. are composed of a lens, a cornea, and an everse retina. The retina contains two different types of cells, ciliary sensory cells and supportive cells which bear one or two cilia. It is suggested that: (a) the ciliary photoreceptors of these teleplanic veliger larvae are correlated with a long pelagic life in the ocean, which can last up to twelve months, and (b) that structural details of the photoreceptors can change during ontogenesis (ciliary vs rhabdomeric). Furthermore, the cilia of the supportive cells apparently tranport lens material and thus play an important role in lens formation. A decomposition mechanism of pigment granules is examined.Abbreviations bb basal body - bp basal plate - c cilium - cc corneal cell - cm ciliary membranes - cw ciliary whorl - ecm extracellular matrix - gr electron-dense granules - l lens - lb lamellar body - mp membranous pieces - mt microtubules - mv microvilli - n nucleus - oc optic cavity - on optic nerve - pg pigment granule - sc sensory cell - sj septate junction - spc supportive cell - v vesicles     

Fine structure of ocelli in larvae of an archiannelid,Polygordius cf. appendiculatus

Summary The ocelli of trochophore and segmented larvae of the archiannelid Polygordius cf. appendiculatus were studied by electron microscopy. An eye consists of two pigmented supportive cells forming an eyecup that encloses one sensory cell bearing one (trochophore) or two (segmented larva) ranks of microvilli and one adventitious cilium. Remarkably abundant tubules (submicrovillar endoplasmic reticulum) radiate from the perinuclear region of the sensory cell, which lies outside the ocellus, toward its receptoral end. Possible functions of the tubules are proposed: carriers of ions, metabolites and photopigments; pinocytic uptake of products resulting from photoreception; storage of membrane; and light guides. Finally, the eyes of Polygordius larvae are believed to have evolutionary significance and provide further support for Eakin's theory of diphyletic origin of photoreceptors.